A three-way catalyst device coupled to the exhaust of a combustion engine reduces combustion by-products such as carbon monoxide, hydrocarbons, and oxides of nitrogen. However, as the catalyst device ages, its ability to store oxygen diminishes leading to a decrease in efficiency. In order to determine the efficiency of the catalyst device, systems monitor the ability of the device to store oxygen.
Various approaches, including full and partial volume monitoring sensor approaches have been developed. In one approach, as described in U.S. Pat. No. 6,594,986, monitoring of the emission control device with a lean-burn engine includes monitoring the oxygen storage capacity of the device by determining an amount of fuel required to purge the catalysts after they have been fully saturated with oxidants due to lean operation.
However, the inventors have recognized issues with such an approach. For example, lean combustion may lead to specific NOx trapping catalyst requirements that may be ineffective for lower emission regulations. Further, in such an approach, since combustion is taking place while the catalysts are saturated for monitoring, there may be a risk of catalyst oversaturation. Catalyst oversaturation may result in decreased reduction of combustion by-products.
As such, the above issued may be addressed at least partially be running a catalyst monitor following deceleration fuel shut-off (DFSO) events, as such operation may provide an advantageous starting point for monitoring the efficiency of the catalysts in an emission control device. In particular, running the monitoring routine following DFSO eliminates the need to operate the engine in a lean combustion mode in order to saturate the catalyst. Further, during DFSO no fuel is injected while the engine rotates and pumps air through the catalyst, thus catalytic saturation may occur faster and more completely than during lean engine operation, with reduced risks from oversaturation.
The inventors herein have also recognized that including compensation for air mass and catalyst temperature variations in the catalyst monitoring routine may reduce noise sources in the catalyst monitoring routine.
In one example, a method is provided for monitoring an emission device coupled to an engine. The method comprises: following a deceleration fuel shut-off duration, indicating degradation of the emission device based on an amount of rich products required to cause a sensor to become richer than a threshold. For example, the sensor may be a full volume sensor located downstream of a full volume of catalyst material. For example, the amount of rich products required to cause a sensor to become richer than a threshold may be correlated to an amount of oxygen stored in the emission device. Thus the indication of degradation of the emission device may be based on the amount of stored oxygen. The indication of emission device degradation may be further based on air mass and temperature during delivery of the required rich products to account for effects of such parameters on the indication of degradation.
In such an approach, the oxygen storage capacity may be identified via an integrated fuel metric. Furthermore, since little to no combustion occurs during DFSO when the catalysts are being saturated, the negative effects of catalyst oversaturation may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.